1. Field of the Invention
The present invention relates to a power supply that outputs a high-voltage using a piezoelectric transformer.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic process, an image formed on a photosensitive member (image bearing member) is transferred on a recording material thereby forming the image on the recording material. In order to transfer the image onto the recording material, the image formed on the photosensitive member is transferred to the recording material by applying a direct current (DC) high-voltage on a transfer roller (transfer member). In order to satisfactorily transfer the image formed on the photosensitive member to the recording material, it is necessary to apply a high-voltage of a several kV or over.
As a power supply apparatus for such a high-voltage output, conventionally a wire-wound electromagnetic transformer has been used. However, the electromagnetic transformer is made up of a copper wire, a bobbin, and a core. When the electromagnetic transformer is used for the image forming apparatus using the electrophotographic process, the leakage current, which leaks from the electromagnetic transformer when a high-voltage is output, must be minimized. For this purpose, insulating the electromagnetic transformer by a mold or the like is absolutely necessary, and accordingly reduction in size and reduction in weight of the power supply apparatus could not be easily realized.
In order to realize such reduction in size and reduction in weight of the power supply apparatus, a power supply apparatus that generates a high-voltage using the piezoelectric transformer is discussed in Japanese Patent Application Laid-Open No. 11-206113.
By using a thin and lightweight piezoelectric transformer including ceramics as raw material, it becomes possible to generate a high-voltage with a higher efficiency than that of the electromagnetic transformer. Further, by using the piezoelectric transformer, it becomes possible to keep a distance between electrodes on primary side and secondary side of the power supply apparatus. As a result, the need for performing a mold processing for insulation, which is necessary in the electromagnetic transformer, can be eliminated. Therefore, with the use of the piezoelectric transformer, the reduction in size and the weight of the power supply apparatus can be realized.
Application of the power supply apparatus, which outputs a high-voltage using the piezoelectric transformer to other than such an image forming apparatus as described above, to various apparatuses that require a high-voltage has been studied.
A configuration for improving workability for attaching the piezoelectric transformer to a substrate, and for reducing a manufacturing cost, when manufacturing the power supply apparatus using such a piezoelectric transformer, is discussed in Japanese Patent Application Laid-Open No. 2006-108332. In Japanese Patent Application Laid-Open No. 2006-108332, the piezoelectric transformer is housed in an upright position within a rectangular box-shaped case, and a terminal of the case and the piezoelectric transformer are connected by a lead wire and soldered, in order to connect electrodes of the piezoelectric transformer and patterns on the printed circuit board.
Further, through the similar configuration to that of Japanese Patent Application Laid-Open No. 2006-108332, a configuration, in which reduction in assembly man-hours, reduction in cost of materials have been realized, is discussed in Japanese Patent Application Laid-Open No. 10-74992. In Japanese Patent Application Laid-Open No. 10-74992, in order to house the piezoelectric transformer in an upright position within a rectangular box-shaped case, and connect electrodes of the piezoelectric transformer and patterns on the printed circuit board, a terminal of the case is connected to the piezoelectric transformer via a conductive rubber, to bring them into conduction. With this configuration, number of assemblyman-hours can be reduced, and reduction in cost becomes possible.
In a configuration using a conductive elastic member (rubber) like the one discussed in Japanese Patent Application Laid-Open No. 10-74992, an allowable current of the conductive rubber is several hundreds milliampere (mA), namely, the allowable current thereof is significantly lower than the allowable current of a lead wire having the order of a few amperes (A), for instance. For this reason, when a DC high-voltage is output as described above, an electric charge is charged from the terminal of a power supply to a capacitance on a primary side of the piezoelectric transformer, and then the electric charge is instantaneously pulled out. As a result, a surge current is generated. The surge current may become a factor responsible for deteriorating the above-described conductive rubber. Therefore, the conductive rubber will be eventually deteriorated by the surge current each time the power supply outputs a high-voltage.
Consequently, for example, like the above-described image forming apparatus, when the piezoelectric transformer is used for an apparatus that needs to output a high-voltage frequently and for long period of time for an image forming, the conductive rubber will be eventually deteriorated. Consequently, it becomes difficult to endure a long-term use. Further, the surge current not only deteriorates the conductive rubber, but also becomes a factor responsible for eventually degrading a peak value of a drive voltage for driving the piezoelectric transformer.